Biaxial artificial disc replacement

ABSTRACT

An artificial disc replacement (ADR) is designed to protect a cushioning component from excessive force. Physical features on the front and back of the ADR enable the device to replicate the normal movements of the spine through predetermined, limited, movements of the endplate components relative to one another. For example, though not limited to these characteristics, the components of the ADR could be dimensioned to allow 15 degrees of flexion, 5 degrees of extension, 5 degrees of lateral bending, and 1-2 mm of translocation. In the preferred embodiment the physical features are axles that extend through overlapping lateral portions associated with the endplate components. A desirable configuration includes a pair of axles, one in the anterior portion and another in the posterior portion, wherein some or all of the axles extend through an oversized aperture that allows the limited relative movement of the endplate components. Alternatively, the anterior and posterior physical features may include mating projections and depressions to permit a desired degree of relative movement.

REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. Provisional PatentApplication Serial No. 60/420,169, filed Oct. 22, 2002, the entirecontent of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to artificial disc replacement(ADR) and, in particular, to ADRs including cushioning components toprotect the implant from excessive force.

BACKGROUND OF THE INVENTION

[0003] Improvements in prosthetic intervertebral disc and jointreplacement components, and related surgical procedures, have led todramatic increases in implant longevity. Many artificial hip and kneecomponents now last for twenty years or more due to improved materialsand greater insight into movement, load distribution and wearcharacteristics.

[0004] Many spinal conditions, including degenerative disc disease, cannow be treated through artificial disc replacement (ADR), which hasseveral advantages over spinal fusion. The most important advantage ofADR is the preservation of spinal motion. Spinal fusion eliminatesmotion across the fused segments of the spine. Consequently, the discsadjacent to the fused level are subjected to increased stress, whichincreases the changes of future surgery to treat the degeneration of thediscs adjacent to the fusion.

[0005] One of the most important features of an artificial discreplacement (ADR) is its ability to replicate the kinematics of anatural disc. ADRs that replicate the kinematics of a normal disc areless likely to transfer additional forces above and below the replaceddisc. In addition, ADRs with natural kinematics are less likely tostress the facet joints and the annulus fibrosus (AF) at the level ofthe disc replacement. Replicating the movements of the natural disc alsodecreases the risk of separation of the ADR from the vertebrae above andbelow the ADR.

[0006] In an attempt to replicate natural disc movements various ADRmaterials have been tried, including hydrogels, metal and rubber. As oneexample, U.S. Pat. No. 6,602,291 resides in a prosthetic spinal discnucleus comprising a hydrogel core surrounded by a constraining jacket.The hydrogel core is configured to expand from a dehydrated state to ahydrated state. In the dehydrated state, the hydrogel core has a shapeselected to facilitate implantation through an annulus opening. Further,in the hydrated state, the hydrogel core has a shape correspondinggenerally to a portion of a nucleus cavity, the hydrated shape beingdifferent from the dehydrated shape. Upon hydration, the hydrogel coretransitions from the dehydrated shape to the hydrated shape.

[0007] Unfortunately, the flexibility of the hydrogel and theconstraining jacket allow hydrogel ADRs to change shape and extrudethrough defects in the annulus through which the ADR was inserted, forexample. My U.S. Pat. Nos. 6,245,107, 6,371,990, 6,454,804, andpublished applications WO 01/10316 A1; 20020156533; 20020165542;20030004574; 20030040796; and 20030078579 are useful in addressing suchproblems.

[0008] Metal and rubber ADRs, on the other hand, also frequently fail atthe metal-rubber interface. The rubber fails directly due to high shearstresses or because the rubber separates from the metal. Clearly anyimprovements in these and other areas would be welcomed by the medicalcommunity and by patients undergoing procedures to implant prostheticcomponents of this kind.

SUMMARY OF THE INVENTION

[0009] This invention broadly resides in an ADR that protects acushioning component from excessive force. In the preferred embodiment,physical features on the front and/or back of the ADR enable the deviceto replicate the normal movements of the spine through predetermined,limited, movements of the endplate components relative to one another.For example, though not limited to these characteristics, the componentsof the ADR could be dimensioned to allow 15 degrees of flexion, 5degrees of extension, 5 degrees of lateral bending, and 1-2 mm oftranslocation.

[0010] In the preferred embodiment the physical features are axles thatextend through overlapping lateral portions associated with the endplatecomponents. A desirable configuration includes a pair of axles, one inthe anterior portion and another in the posterior portion, wherein someor all of the axles extend through an oversized aperture that allows thelimited relative movement of the endplate components. Alternatively, theanterior and posterior physical features may include mating projectionsand depressions to permit a desired degree of relative movement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a view of the lateral aspect of the spine and an ADRaccording to the present invention;

[0012]FIG. 2 is a view of the anterior aspect of the spine and the ADR;

[0013]FIG. 3 is a view of the lateral aspect of a flexed spine and theADR;

[0014]FIG. 4 is a view of the anterior aspect of a spine in lateralbending and the ADR;

[0015]FIG. 5A is a coronal cross section of one configuration whereinfour axles connect the endplates;

[0016]FIG. 5B is a coronal cross section of an alternative embodiment;

[0017]FIG. 5 shows how the component nests in each other;

[0018]FIG. 6A is a view of the lateral aspect of the ADR with optionalscrews to fix the ADR to the vertebrae;

[0019]FIG. 6B is a frontal view with an emphasis on screw orientation;

[0020]FIG. 7A is a view of the lateral aspect of an alternativeembodiment of the ADR;

[0021]FIG. 7B is a view of the top of the ADR drawn in FIG. 7A;

[0022]FIG. 8 is a view of the lateral aspect of an alternativeembodiment of the ADR drawn in FIG. 7A;

[0023]FIG. 9A is a view of the lateral aspect of the spine and thesuperior endplate of the ADR;

[0024]FIG. 9B is a view of the lateral aspect of the spine and theinsertion of the inferior endplate;

[0025]FIG. 9C shows insertion of the elastomeric component;

[0026]FIG. 9D shows insertion of the anterior plate onto the inferiorendplate of the ADR;

[0027]FIG. 10A is a view of the lateral aspect of another embodiment ofthe present invention;

[0028]FIG. 10B is a view of the anterior aspect of the device drawn inFIG. 10A;

[0029]FIG. 11A is a view of the lateral aspect of yet a furtheralternative embodiment of the ADR; and

[0030]FIG. 11B is a view of the lateral aspect of the device drawn inFIG. 11A.

DETAILED DESCRIPTION OF THE INVENTION

[0031]FIG. 1 is a view of the lateral aspect of the spine and an ADRaccording to the invention depicted generally at 100. A top endplatecomponent 102 articulates with a bottom endplate component 104 throughaxles 106, 108 in the front and back of the ADR. A cushioning component120 is disposed between the endplate components 102, 104. Holes 110, 112of the bottom endplate component receive the axles 106, 108 and aresized to allow normal movements of the spine, preferably a limitedamount of flexion, extension, lateral bending, and/or translocation.Depending upon the type and degree of desired movement, over-sized holesmay be provided on the top endplate component, or both components 102,104.

[0032] The endplate components are preferably made of metal, and thesurface of each endplate component adjacent to the vertebrae wouldpreferably incorporate a bone-ingrowth promoting surface of the typesknown to those skilled in orthopaedic design. The cushion component 120would likely be an elastomer, though contained hydrogels and othercompressible synthetic and natural members may alternatively be used.The invention is not limited in this regard. The cushion component 120would be held in position between the endplate components 102, 104 bylips around the periphery of the endplate components. As such, thecushioned 102 component would not need to be bonded to the endplatecomponents.

[0033]FIG. 2 is a view of the anterior aspect of the spine and the ADR.Note that the various components are preferably sized to facilitatelateral bending. FIG. 3 is a view of the lateral aspect of a flexedspine and the ADR, better reveling how the anterior axle travels to theinferior aspect of the hole in the bottom endplate component. Theposterior axle travels to the top of the posterior hole in the bottomendplate component. FIG. 4 is a view of the anterior aspect of a spinein lateral bending and the ADR. As discussed above, the top endplatecomponent could impinge against the bottom endplate component after alevel of lateral bending such as 5 degrees.

[0034] The articulations between the two-endplate components 102,104limit the forces experienced by the cushion component 120. For example,the cushion component 120 may be protected from excessive shear. Thedimensions of the endplate components 102, 104 may further bedimensioned and/or proportioned to protect the cushion component 102from excessive axial loads. As best seen in FIG. 4, the top and bottomendplate components 102, 104 may be designed to impinge after a certainamount of compression of the cushion component 120. Such impingementbetween the top and bottom endplate components would also help toprotect the axles 106,108. In addition, impingement of the endplatecomponents 102, 104 should help protect the facet joints and remainingannulus fibrosis from excessive force.

[0035] The axles may or may not extend through the cushion component.FIG. 5A is a coronal cross section of one configuration wherein fouraxles connect the endplates. FIG. 5B is a coronal cross section of analternative embodiment wherein two axles connect the endplates. Notethat although separate elements are shown for the endplate componentsand axles, the axles may be integral to one of the components,particularly the component that nests in the other, as shown in FIG. 5C.

[0036]FIG. 6A is a view of the lateral aspect of the ADR with optionalscrews to fix the ADR to the vertebrae. FIG. 6B is a front view. In thepreferred embodiments, the ADR would incorporate some mechanism toprevent the screws form backing out.

[0037]FIG. 7A is a view of the lateral aspect of an alternativeembodiment of the ADR. FIG. 7B is a view of the top of the ADR drawn inFIG. 7A. In this case, hemi-cylindrical projections 702 from the upperendplate articulate with hemi-piston projections 704 form the lowerendplate. As described above the articulations between the endplatespermit normal spinal movement and cushioning. The endplates alsoprohibit excessive movements and excessive forces between the endplates.

[0038]FIG. 8 is a view of the lateral aspect of an alternativeembodiment of the ADR drawn in FIG. 7A, including a removable plate 802associated with the lower endplate allows assembly of the ADR within thedisc space. The shape of the endplates, the articulations between theendplates, and the removable plate hold the elastomer in place betweenthe endplates. Again, the elastomer need not be glued to the endplates.

[0039] FIGS. 9A-9D illustrate an insertion sequence applicable to theADR drawn in FIG. 8. FIG. 9A is a view of the lateral aspect of thespine and the superior endplate 804 of the ADR. Spikes 806 on theendplate are press fit into the vertebral endplate 810 using adistraction instrument 812. FIG. 9B is a view of the lateral aspect ofthe spine and the insertion of the inferior endplate 814. Thedistraction instrument 812 fits into the upper and lower ADR endplatesto assure the inferior endplate is properly aligned with respect to thesuperior endplate of the ADR. FIG. 9C shows insertion of the elastomericcomponent. FIG. 9D shows insertion of the anterior plate onto theinferior endplate of the ADR. A screw or screws holds the anterior plateon the in inferior endplate of the ADR.

[0040]FIG. 10A is a view of the lateral aspect of another embodiment ofthe invention. FIG. 10B is a view of the anterior aspect of the devicedrawn in FIG. 10A. A lip 1002 on the posterior aspect of the inferiorendplate 1004 prevents the superior endplate 1006 from sliding forward.The tongue-like projections 1008, 1010 of the superior and inferiorendplates cooperate to limit lateral bending, for example to 5 degreesin either direction. The posterior projections form the superiorendplate can also limit extension; to 5 degrees, for example. A movableclip 1020 prevents the elastomer 1022 from falling out of the front ofthe device. The elastomer may have a thin sheet of metal glued to itssuperior and inferior surfaces. The low friction of the metal sheetswould facilitate movement of the elastomer between the endplatecomponents.

[0041]FIG. 11A is a view of the lateral aspect of yet a furtheralternative embodiment of the ADR. The projections from the inferiorendplate have retractable extensions. The extensions 1102,1104 help holdthe elastomer in position without interfering with the motion of the twoendplate components. The endplate components do not cooperate to limitlateral bending in this embodiment. The dotted line represents theoutline of the portion of the retractable projection that lies within arecess of the projection from the inferior endplate. FIG. 11B is a viewof the lateral aspect of the device drawn in FIG. 11A. The retractableprojections are drawn in an elevated position.

I claim:
 1. An artificial disc replacement (ADR) configured forplacement between two vertebral bodies having a medial-lateralorientation and anterior and posterior portions, the ADR comprising:opposing endplate components, each fixed to a respective one of thevertebral bodies; a cushioning component disposed between the opposingendplate components; and anterior and posterior features that permit apredetermined, limited, movement of the endplate components relative toone another.
 2. The ADR of claim 1, wherein the anterior and posteriorfeatures permit a predetermined, limited degree of flexion.
 3. The ADRof claim 1, wherein the anterior and posterior features permit apredetermined, limited degree of extension.
 4. The ADR of claim 1,wherein the anterior and posterior features permit a predetermined,limited degree of lateral bending.
 5. The ADR of claim 1, wherein theanterior and posterior features permit a predetermined, limited degreeof translocation.
 6. The ADR of claim 1, wherein: the opposing endplatecomponents have overlapping medial and lateral portions; and thefeatures are axles that extend through the overlapping lateral portions.7. The ADR of claim 6, including a pair of axles, one in the anteriorportion and another in the posterior portion.
 8. The ADR of claim 6,wherein one of the axles extends through an oversized aperture formed inan overlapping portion that allows the limited relative movement of theendplate components.
 9. The ADR of claim 1, wherein: the opposingendplate components have overlapping lateral portions; and the anteriorand posterior physical features include mating projections anddepressions associated with the overlapping lateral portions.